tomcat   继承错误

我要继承tomcat 的WebdavServlet 类，
package com.ponder.web;


import java.io.IOException;
import java.util.Enumeration;

import javax.servlet.ServletException;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

import org.apache.catalina.servlets.DefaultServlet;
import org.apache.catalina.servlets.WebdavServlet;


public class MyWebDavServlet extends WebdavServlet{

/**
*
*/

private static final long serialVersionUID = 1L;
public void init()  throws ServletException{
System.out.println("OK-Init");
super.init();
}
@Override
protected void service(HttpServletRequest request,  HttpServletResponse response) throws ServletException,IOException{


System.out.println("process start : " + request.getRequestURL().toString());
System.out.println("OK-Init");
super.service(request, response);
System.out.println("process over : " + request.getRequestURL().toString());

}


}
发布到tomcat起动正常
一访问就出报出错误如下
OK-Init
2008-11-16 10:07:03 org.apache.catalina.core.ApplicationContext log
严重: StandardWrapper.Throwable
java.lang.ClassCastException: org.apache.naming.resources.ProxyDirContext cannot be cast to org.apache.naming.resources.ProxyDirContext
at org.apache.catalina.servlets.DefaultServlet.init(DefaultServlet.java:256)
at org.apache.catalina.servlets.WebdavServlet.init(WebdavServlet.java:263)
at com.ponder.web.MyWebDavServlet.init(MyWebDavServlet.java:25)
at javax.servlet.GenericServlet.init(GenericServlet.java:212)
at org.apache.catalina.core.StandardWrapper.loadServlet(StandardWrapper.java:1161)
at org.apache.catalina.core.StandardWrapper.allocate(StandardWrapper.java:806)
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:129)
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:175)
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:128)
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:102)
at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:109)
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:286)
at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:844)
at org.apache.coyote.http11.Http11Protocol$Http11ConnectionHandler.process(Http11Protocol.java:583)
at org.apache.tomcat.util.net.JIoEndpoint$Worker.run(JIoEndpoint.java:447)
at java.lang.Thread.run(Unknown Source)
2008-11-16 10:07:03 org.apache.catalina.core.StandardWrapperValve invoke
严重: Allocate exception for servlet webdav
java.lang.ClassCastException: org.apache.naming.resources.ProxyDirContext cannot be cast to org.apache.naming.resources.ProxyDirContext
at org.apache.catalina.servlets.DefaultServlet.init(DefaultServlet.java:256)
at org.apache.catalina.servlets.WebdavServlet.init(WebdavServlet.java:263)
at com.ponder.web.MyWebDavServlet.init(MyWebDavServlet.java:25)
at javax.servlet.GenericServlet.init(GenericServlet.java:212)
at org.apache.catalina.core.StandardWrapper.loadServlet(StandardWrapper.java:1161)
at org.apache.catalina.core.StandardWrapper.allocate(StandardWrapper.java:806)
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:129)
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:175)
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:128)
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:102)
at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:109)
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:286)
at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:844)
at org.apache.coyote.http11.Http11Protocol$Http11ConnectionHandler.process(Http11Protocol.java:583)
at org.apache.tomcat.util.net.JIoEndpoint$Worker.run(JIoEndpoint.java:447)
at java.lang.Thread.run(Unknown Source)。
web.xml如下
<?xml version="1.0" encoding="UTF-8"?>

<!DOCTYPE web-app
    PUBLIC "-//Sun Microsystems, Inc.//DTD Web Application 2.3//EN"
    "http://java.sun.com/dtd/web-app_2_3.dtd">

<web-app>
    <servlet>
  <servlet-name>webdav </servlet-name>
  <servlet-class>com.ponder.web.MyWebDavServlet </servlet-class>
    <init-param>
      <param-name>debug </param-name>
      <param-value>0 </param-value>
    </init-param>
    <init-param>
      <param-name>listings </param-name>
      <param-value>true </param-value>
    </init-param>
  </servlet>
  <servlet-mapping>
    <servlet-name>webdav </servlet-name>
    <url-pattern>/* </url-pattern>
  </servlet-mapping>
</web-app>

请问我要如何解决哎